worker_num to worker_id (#238)

docs/Unity-Agents---Python-API.md

 
 ```python
 from unityagents import UnityEnvironment
-env = UnityEnvironment(file_name=filename, worker_num=0)
+env = UnityEnvironment(file_name=filename, worker_id=0)
-* `worker_num` indicates which port to use for communication with the environment. For use in parallel training regimes such as A3C.
+* `worker_id` indicates which port to use for communication with the environment. For use in parallel training regimes such as A3C.
 
 ## Interacting with a Unity Environment